1. Field of the Invention
The present invention relates to phased array antennas and, more particularly, relates to dynamic plasma driven phased array antennas.
2. Description of the Prior Art
Various phased array antenna configurations have been employed in the prior art. One such antenna configuration is disclosed in U.S. Pat. No. 4,905,014 to Gonzalez et al., entitled "Microwave Phasing Structures For Electromagnetically Emulating Reflective Surfaces And Focusing Elements Of Selected Geometry," issued on Feb. 27, 1990, the disclosure of which is incorporated herein by reference.
The antenna configuration disclosed in the Gonzalez et al. patent includes an electrically thin microwave phasing structure for electromagnetically emulating a desired reflective surface of selected geometry over an operating frequency band. The microwave phasing structure includes a support matrix, i.e., a dielectric substrate, and a reflective means, i.e., a ground plane, for reflecting microwaves within the frequency operating band. The reflective means is supported by the support matrix. An arrangement of electromagnetically-loading structures is supported by the support matrix at a distance from the reflective means which can be less than a fraction of the wavelength of the highest frequency in the operating frequency range. The electromagnetically-loading structures are dimensioned, oriented, and interspaced from each other and disposed at a distance from the reflective means, as to provide the emulation of the desired reflective surface of selected geometry. Specifically, the electromagnetically-loading structures form an array of metallic patterns, each metallic pattern preferably being in the form of a cross, i.e., X configuration. It is disclosed that each electromagnetically-loading structure can be constructed to form different geometrical patterns and, in fact, could be shorted crossed dipoles, metallic plates, irises, apertures, etc. It is further disclosed that the microwave phasing structures of the Gonzalez et al. patent may be used for electromagnetically emulating a desired microwave focusing element of a selected geometry.
The selected geometry of the desired reflective surface can be a parabolic surface in order to emulate a parabolic reflector wherein all path lengths of the reflected incident electromagnetic waves are equalized by phase shifting affected by the microwave phasing structure of the present invention. While the microwave phasing structure may emulate desired reflective surfaces of selected geometries such as a parabola, the microwave phasing structure is generally flat in shape. However, the shape of the microwave phasing structure may be conformal to allow for mounting on substantially non-flat surfaces.
It is to be appreciated that the phased array antenna technology disclosed in the Gonzalez et al. patent is commonly owned by the assignee of the present invention (Malibu Research Associates, Inc. Of Calabasas, Calif.) and is generally referred to as FLAPS.TM. technology.
Referring now to FIGS. 1A through 1D, an exemplary embodiment of an electromagnetically-loading structure (Fig. 1A) formed in accordance with the FLAPS.TM. technology as disclosed in the Gonzalez et al. patent, and arrays thereof (FIGS. 1B through 1D), are shown. The basic elemental structure, as shown in FIG. 1A, is a crossed shorted dipole situated over a ground plane with an intermediate dielectric material sandwiched therebetween. It is to be appreciated that each arm of the crossed dipole independently controls its corresponding polarization. Incident RF (radio frequency) energy causes a voltage standing wave to be set up between the dipole and the ground plane. The dipole itself possesses an RF reactance which is a function of the size of the dipole. This combination of the formation of a voltage standing wave and the dipole reactance causes the incident RF energy to be reradiated with a phase shift .phi..
The exact value of this phase shift .phi. is a complex function of the dipole length and thickness, the distance between the dipole and the ground plane, the dielectric constant associated with the dielectric spacer and the angle associated with the incident RF energy. When used in an array, as shown in FIG. 1B through 1D, the phase shift .phi. associated with a dipole is also affected by nearby dipoles.
In practice, the dipole arm lengths may be within the approximate range of one-quarter (1/4) to one-sixteenth (1/16) of the wavelength of the operating frequency of the incident RF energy in order to provide a full range of phase shifts. The preferred spacing between a dipole and the ground plane is between approximately one-sixteenth (1/16) and one-eighth (1/8) of the wavelength associated with the incident RF energy wave. It is to be appreciated that the dipole/ground plane spacing also affects certain parameters of the phased array antenna, such as form factor, bandwidth and sensitivity to fabrication errors. The dipole structure in FIG. 1A is typically formed by the etching of a printed circuit board. At longer wavelengths (i.e., lower incident RF energy operating frequencies), plating of a dielectric fiber strand is an alternate dipole fabrication method. It is to be appreciated that a radiating element formed in accordance with the FLAPS.TM. technology may operate at frequencies in the microwave and millimeter wave range.
As shown in FIG. 1B, each radiating element functions in a similar manner as a static phase shifter in a phased array antenna. Specifically, if a plurality of such radiating elements are designed to reradiate incident RF energy with a progressive series of phase shift .phi., 2.phi., 3.phi.. . . n.phi., then a resultant RF beam is formed in the direction .theta., which may be represented as: ##EQU1## where d.sub.x represents the spacing between radiating elements, .lambda. represents the wavelength of the incident RF energy and .phi. represents the element-to-element phase shift, i.e., the phase gradient.
Equation (1) is for beam steering in a single plane. Just as in two-dimensional phased array antennas, beam steering can be accomplished in both azimuth and elevation by application of phase gradients among the dipole radiating elements in both the x and y planes. In such case, the beam scan equation is dependent upon both the x and y spacings of the elements. It is to be appreciated that while the angle .theta. is referred to as the scan angle, the phased array formed by the radiating elements described in the Gonzalez et al. patent performs beam steering and focusing only, that is, the incident RF energy is reradiated in a single direction .theta., depending on the formation of the radiating elements, and does not perform an electronic scanning function.
While the embodiment illustrated in FIG. 1A shows a zero degree angle of incident RF energy, the incident RF wave may, in fact, be at any angle up to approximately 70 degrees. When such is the case, the angle of scattered energy, .theta., may be more generally represented as: ##EQU2## where .theta..sub.o is the angle of incidence and .theta. is the beam energy scattering angle. Note that if: ##EQU3## then the RF energy is returned in the direction from which it came even though the surface containing the radiating elements is at a tilted angle.
The phased array described in the context of FIG. 1B is considered to perform uniform radiation beam steering. However, this concept may be extended to the situation in which either the steering angle .theta. or the angle of incidence .theta..sub.o, or both, are adjusted over the surface of the phased array of radiating elements. Such an approach, which utilizes a flat collimating surface, is illustrated in FIG. 1C. In the approach shown in FIG. 1C, the steering angle developed by the phase shifts of each radiating element is set in order to cause all incident energy to be focused on a feed. In this manner, the phased array functions as a parabolic reflector, but in a flat surface configuration. As shown in FIG. 1C, the RF energy is both focused and steered toward an offset feed. Using the above described local steering properties further allows the surface to be conformed to any reasonably smooth shape. Such a conformal phased array is illustrated in FIG. 1D.
While the above-described phased array antennas, formed utilizing the FLAPS.TM. technology disclosed in the Gonzalez et al. patent, permit emulation of reflective surfaces and focusing elements of selected geometry, the individual radiating elements, e.g., dipoles, cannot be dynamically reconfigured. Due to the lack of dynamic reconfigurability of the dipoles, the above-described phased array antennas are incapable of dynamically varying the phase shifts associated with the dipoles and, therefore, such antennas cannot perform electronic scanning functions.